I don't know how Ubuntu's installer looks like, but if you have
unallocated space in the disk it should be detected.
"/dev/sda" is the whole disk (not a Windows partition), while "/dev/
sda1", "/dev/sda2", "/dev/sda3" and so on... name the partitions in the
disk. Maybe you have to manually select the partitions or tell the
installer to look in another place (under some kind of "advanced
settings"?).
I have installed XP and Debian without a problem. But the other day I
needed to do exactly this, install Ubuntu 10.10 on a dual boot with
Windows. It refused to see the empty space, although I tried every
method I could think of. Because the 10.10 installation was urgent, I
wiped the XP off the disk and let 10.10 start from scratch. Annoying,
as XP had taken me a day on and off to install, was a right pain, and I
had hoped to leave it alone. I also had 10.10 refuse to quadrupal boot
on another machine with Debian and 2 other versions of Ubuntu.
I keep hoping that Ubuntu and I will make peace. Then something like
this happens and I flee back as soon as I can to Debian.
But 10.10 is great for getting a desktop rapidly to the point of using
multimedia to the satisfaction of my granddaughter. For everything else
I find it a pain. :-(